Power amplifiers have a wide range of application in modern electronics, particularly in communication systems. Power amplifiers are often included in transmitters to amplify signals before transmission. FIG. 1A is a block diagram illustrating a single-ended power amplifier example. The power amplifier shown in this example includes a first amplification stage 100 and a second amplification stage 102. The amplified signal is to be transmitted via antenna 104. Antenna 104 may experience a significant amount of electro-static discharges because it is exposed to the environment. In this single-ended configuration, since the antenna is directly connected to the output of the second stage amplifier, the electro-static discharges may negatively impact the performance and functionality of the second stage amplifier. In a cellular phone system, for example, the voltage swing at the output of the second stage amplifier may exceed the power supply voltage, possibly causing damage to integrated circuit (IC) devices and leading to long term reliability issues. It is desirable for the second stage amplifier to maintain its functionality and performance under potentially harsh conditions. Additionally, a single-ended amplifier is susceptible to power supply noise. A regulator is commonly used to reduce the power supply noise and fluctuation.
FIG. 1B is a block diagram illustrating a differential power amplifier example. Differential power amplifier 160 shown in this example includes a first amplification stage 150 and a second amplification stage 152. Differential amplifiers have better power supply rejection performance than single-ended amplifiers and can more effectively reduce power supply noise. Since antenna 154 is a single-ended device, a transformer 156 (also referred to as a balun) is used to convert the balanced output of the double ended amplifier to an unbalanced signal to be transmitted by antenna 154. Balun 156 electrically isolates the output of the second stage from the antenna and ameliorates the problems associated with electro static discharges. Although the design shown in FIG. 1B is useful for noise reduction and electrical isolation from the antenna, some problems still remain in its implementation. The coils of the inductors in balun 156 introduce resistance, which is especially significant when the inductors are implemented in a monolithic form such as spiral inductors in IC devices. Multiple loops in the inductive coil are usually required in order to achieve the desired inductance. As the length and resistance of the inductor increases, the number of loops increases, leading to greater power dissipation and reduced overall system efficiency. The resistance introduced by the inductive coils is particularly problematic for IC devices since power consumption is typically an important consideration, especially for battery operated devices. It would be desirable to have an amplifier design that would provide good power efficiency. It would also be useful if the amplifier can be used in transmitter circuits and provide good isolation between the amplification stages and the antenna.